Project II: Drug-Drug Interactions Between Pharmaceutical and Endogenous Antibiotics Antimicrobial peptides (AMPs) are natural antibiotics produced by leukocytes, epithelial cells, and platelets in response to infection. Our laboratory has studied the critical role of endogenous AMPs in defense against invasive bacterial infection, and characterized mechanisms by which leading bacterial pathogens display relative resistance to AMP killing. Their significance of AMPs in innate defense is magnified in the youngest age groups because of quantitative and qualitative defects in antibody and cellular immune responses. Recently we have discovered striking evidence of interactions between common antibiotics used in clinical practice and endogenous AMPs - such that particular antibiotics markedly decrease or increase bacterial AMP susceptibility. Effectively, different antibiotics appear to work in concert with, or at odds with, with our own innate immune system in killing important bacterial pathogens such as Staphylococcus aureus. For example, showed that bacteriostatic antibiotics including erythromycin, TMP-SFX, or chloramphenicol markedly reduced the susceptibility of several Gram+ or Gram- bacterial pathogens to diverse AMPs, resulting in true antagonism. In other research, we have identified novel mechanisms by which different beta-lactam antibiotics can increased the susceptibility of drug-resistant pathogens like MRSA and VRE to AMPs, resulting in true synergy. We hypothesize that interactions between pharmaceutical and endogenous antibiotics have important pharmacodynamic effects and implications for clinical infectious disease therapy. Thus this project proposes an entirely new approach in antibiotic pharmacology: the careful analysis of potential drug:drug interactions between pharmaceutical antibiotics and critical host AMPs in therapy of common bacterial infections in children . In Aim 1 will identify synergistic and antagonistic drug:drug interactions between leading pharmaceutical antibiotics and four distinct human AMPs. against leading pediatric bacterial pathogens. In Aim 2, we define developmental and organ-specific parameters of the endogenous pharmacodynamic response of cathelicidin AMPs. In Aim 3, we probe the mechanistic basis for antibiotic:AMP synergy vs. drug-resistant Gram+ pathogens. This project has the potential to shift paradigms in a way that is immediately translational to the clinical setting, since we are studying drug:drug interactions between widely used clinically approved antibiotics with those antibiotics that nature gave us.